Proof of treatment closures for vessels and methods of use of the closures are described. Embodiments the closures indicate that vessel contents have reached at least a predetermined temperature. The proof of treatment closures can show that vessel contents a distance within the vessel interior have reached the threshold conditions, rather than reflecting conditions of the vessel itself, or of vessel contents in close proximity to a vessel peripheral boundary. Embodiments include proof of treatment closures resistant to status tampering, such that it is difficult to activate the indicator without reaching the specified conditions. The proof of treatment closures can also be seal tamper evident, such that loosening or removing the closure from the vessel, or otherwise breaking a seal between the closure and the vessel, leaves conspicuous evidence of such.